This invention relates generally to mechanical face seals for sealing the space between a rotating shaft and its housing and in particular to face seals having grooves in their sealing surfaces for maintaining a gap between the relatively rotating sealing members.
Spiral groove mechanical face seals are used to create a non-contacting seal between a rotating shaft and its housing. A gap between two sealing faces, one of which is rotating with regard to the other, is maintained by a film of pressurized fluid pumped between the faces by spiral grooves in at least one of the sealing faces. Examples of spiral groove face seals are found in U.S. Pat. No. 3,804,424 issued to Gardner and U.S. Pat. Nos. 4,212,475 and 4,290,611 issued to Sedy. All of these patents are commonly assigned to the assignee of the present invention.
The spiral groove face seals described in these patents and generally those in present use include an annular primary sealing ring having a radially extending face in sealing relation with the radially extending face of an annular mating ring. In operation, either the primary ring or the mating ring rotates with the shaft and includes a radial face in sealing relation to the radial face of the other ring which is itself sealed against the housing. In conventional contacting seals, friction between the two faces during relative rotation produces heat, causing seal face deformation, accelerated aging of the seal components and other undesirable conditions.
It has been found that a very narrow gap or space between the radial faces permits some of the sealed fluid to leak or flow to the low pressure side and that the fluid flow prevents unwanted heat generation. Such a gap is obtained by a series of spiral grooves in the face of either the primary or mating ring which, upon rotation of one of the rings, act as a pump to force fluid between the seal faces. The fluid flow separates the faces and acts as a lubricant, maintaining the gap and allowing the faces to slide against one another without contact between them.
Other mechanical face seals utilizing grooved surfaces have been proposed. For example, U.S. Pat. No. 4,420,162 describes a face seal having spiral grooves extending from the inner circumference to the outer circumference that are both forwardly and rearwardly inclined with respect to the direction of rotation of the seal face. One set of either forwardly or rearwardly inclined grooves acts to pump the sealed fluid out of the gap between seal faces while the oppositely inclined set of grooves acts to pump fluid into the gap.
These seal face designs, however, do not provide the ideal sealing structure. The seal face spiral groove structure described in U.S. Pat. No. 4,420,162 pumps fluid through the seal in a contacting seal face environment.
Seal faces pumping fluid in only one direction provide a fluid film thickness between the faces that is excessive and results in unwanted and unnecessary leakage. The leakage is somewhat reduced if there is a sealing dam adjacent either the inner or outer diameter of a sealing ring. A dam is an ungrooved annular surface adjacent to the grooved annular surface Moreover, the asymmetry in the spiral direction of the prior art seal faces permit their rotation in only one direction so as to produce their intended gap creating function. Rotation of the shaft in the opposite direction or improper installation of the sealing rings creates a vacuum instead of a gap, and operation of the equipment can seriously damage one or both of the seal faces.
As is recognized by Sedy in U.S. Pat. No. 4,212,475, it is desirable to make the fluid film thickness as small as possible to reduce leakage while simultaneously increasing the film stiffness and thus providing stability to the seal faces and gap dimension. The solution proposed by Sedy provides for specific parameters in the length, width and thickness of the grooves relative to the dimensions of the lands and the dam. This solution works well enough when fluid leakage is not a problem, but will nevertheless produce more excess leakage than is necessary.